dd: Check result of close(2) for errors

close(2) can return errors from previous operations which should not be ignored.

PR: 229616
Submitted by:	Thomas Hurst
Reported by:	Thomas Hurst
Reviewed by:	mmacy@
Obtained from:	Ryan Moeller
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D21376

bin/dd/dd.c

@@ -124,7 +124,8 @@ main(int argc __unused, char *argv[])
 	 * descriptor explicitly so that the summary handler (called
 	 * from an atexit() hook) includes this work.
 	 */
-	close(out.fd);
+	if (close(out.fd) == -1 && errno != EINTR)
+		err(1, "close");
 	exit(0);
 }